Impact of management components, abiotic factors and soil biological activity on carbon stocks in temperate forests

Forests are pivotal in regulating the global carbon cycle, yet the complex interplay among forest management practices, abiotic conditions, and soil biological activity in shaping carbon stocks remains insufficiently understood. This study addresses parts of this knowledge gap by disentangling the influence of certain abiotic, biotic and management factors on above- and belowground carbon stocks in temperate forests across four ecoregions in north-western Germany, differing in site conditions and forest communities. In each region, 50 plots were established along a gradient of management intensity, ranging from beech or oak reserves to conifer-dominated stands. Carbon stocks were quantified in aboveground biomass, roots, deadwood, and the organic layer, as well as in mineral soil at depths of 0–5, 5–30 and 30–60 cm. We show that aboveground carbon storage was primarily determined by forest management, while soil organic carbon (SOC) stocks were mainly controlled by abiotic site conditions. An increased proportion of conifers was associated with higher SOC stocks in the organic layer (∼ 1.8 Mg SOC ha −1 per +10 % conifers). However, these thick organic layers are vulnerable to rapid decomposition following canopy disturbances such as windthrow or fire, whereas long-term stable carbon storage is primarily provided by the mineral soil. Significant lower SOC stocks (average 23 Mg ha −1 ) were observed in mineral soils of formerly limed plots. Structural equation modeling indicated that a greater share of deciduous trees affects SOC stocks both directly and indirectly. Contrary to the direct negative effect on SOC stocks, increasing broadleaves enhance soil biological activity, which in turn promoted the transfer of organic carbon from the labile organic layer to the more stable mineral soil pool. Although targeted management can influence SOC, its overall effect is relatively minor compared to that of abiotic factors. • Aboveground carbon stocks are primarily determined by management. • Management has overall little effect on soil organic carbon (SOC) stocks. • SOC stocks are mainly controlled by abiotic factors (stone content and moisture). • Liming reduces SOC stocks in organic layer and mineral soil. • Broadleaves enhance microbial growth and thus indirectly SOC stocks in mineral soil.